Abstract: Aims of this study were to synthesize methoxy poly (ethylene glycol)-b-poly (ε-caprolactone) diblock copolymers (MPEG-b-PCL) and to prepare their surfactant-free nanospheres by modified-spontaneous emulsification solvent diffusion method for controlled release delivery of hydrophobic drug. Ibuprofen was used as a poorly-water soluble model drug. Influences of PCL block length and drug loading content on nanosphere characteristics and drug release behaviors were evaluated. The prepared nanospheres with and without drug loading were found of spherical shape and smooth surface with size less than 150 nm. FTIR spectroscopy and differential scanning calorimetry studies showed the interactions between ibuprofen and MPEG-b-PCL had occurred, suggesting the well distribution of ibuprofen into nanosphere matrices. An increase in the PCL block length led to a decrease in drug encapsulation efficiency. The drug release profiles were biphasic with a fast burst release followed by a slow one. The drug release rates from the MPEG-b-PCL nanospheres strongly depended on the PCL block length and the drug loading content.